The present invention relates to a device for displaying images transmitted in the form of an electric signal, commonly called a video signal. Among the various possible display devices, the invention more particularly applies to matrix-type, high definition liquid crystal display screens. It relates to the matrix sweep or scan circuit organized so as to obtain a high definition by means of a smaller number of sweep control devices.
The invention more particularly applies to display screens incorporating liquid crystals in the smectic phase and which are current-controlled.
It is known that on cooling a thin layer of material having a smectic phase and on starting from the liquid phase, the optical appearance of the thin layer significantly depends on the cooling rate. If cooling takes place slowly, the material is uniformly oriented and the layer appears perfectly transparent. However, if the transition from the liquid phase to the smectic phase takes place very rapidly, areas form in the layer having different orientations from one another and leading to a high diffusion of the transmitted or reflected light.
It is known to use this effect for recording an image on a liquid crystal film having a smectic phase. The material, which is placed between two transparent plates, is kept at a temperature such that it is in its smectic phase, the recording of an image point being obtained by heating the liquid layer followed by rapid cooling. It is known to supply the necessary heat quantity for melting the liquid crystal layer by infrared radiation or laser radiation.
On May 5th 1977, the present Applicant Company filed French Patent Application No 77,13 738 according to which it is possible to increase the image recording speed in a layer of material having such a thermoelectric effect by using filament resistors making it possible to record an image line-by-line, so that then the duration of one line is available for recording simultaneously all the points of said line. This recording and erasure process is faster than optical processes and makes it possible to come close to recording video images on a display screen.
However, the design of high definition display screens according to this process comes up against a problem linked with the number of lines and columns of the matrix of points defining the image, said higher number corresponding to a high number of electronic control devices for the line-by-line sweep.
Hereinafter, the term control electronics is used to define all the devices making it possible to pass current into a line of the screen matrix, said device more particularly comprising one or two power transistors, the transistors which control their bases and the clock systems, shift registers and other sequencers.
The various known matrix display panels and display screens, no matter whether they use light-emitting diodes, liquid crystals, plasma or are electrolytic are confronted with the construction of the external control electronics and particularly the price thereof. Thus, a high definition display screen, having for example 512 or 1024 lines or columns corresponding essentially to the definition of a television image according to existing standards, thus assumes the association of 512 or 1024 connections to 512 or 1024 external control electronics, which is very costly in practice. Such cabling using wires or multicore flat cables in accordance with the prior art can be performed in the laboratory, but does not form an industrial solution. This is made worse by the fact that the cost of the control electronics is actually higher than that of the display screen.